This invention concerns the field of cooling a rotating electrical machine particularly a high speed reluctance machine that is free of rotor windings.
The switched-reluctance electro machine, i.e., the reluctance generator/motor, is presently considered desirable for use as a direct-drive energy transducer for coupling the rotatable shaft of a turbine engine to either an electrical load or to an electrical energy source. A significant example of this usage appears in the relatively small and lightweight apparatus providing modern aircraft with stand-alone ground electrical energy and air conditioning, for examples, and with in-flight engine restart capability. In such equipment a switched reluctance generator/motor may be used to both generate electrical energy and provide auxiliary power unit turbine engine starting torque and main engine starting torque.
Use of such an electrical to mechanical transducer in a larger size as both the starting motor and the electrical generator coupled to the propulsion engine of an aircraft is also possible. The switching used in such machines is preferably accomplished with electronic devices such as transistors operated in synchronism with positioning of the machine rotor. Generally a given machine is considered to provide motor action when its rotor to stator pole gap is decreasing under the influence of winding current flow and generator action when pole overlap is increasing. The electrical switching is used to exclude or minimize an unwanted of these two effects in a particular use application with switching events occurring as fast as each 250 nanoseconds. When combined with switching circuitry, the reluctance motor/generator is often referred-to using the name "switched reluctance motor/generator" or "variable reluctance machine". Switching excitation is of course appropriate when the machine is used as a motor and is also used in connection with the machine functioning as a generator. According to refinements of such switching it can also be used to preclude pole tip saturation and related heating effects in the operation of some reluctance motor/generator machines.
The arrangement of apparatus for stand-alone ground electrical energy generation and air conditioning purposes has evolved toward a relatively high speed hot gas turbine directly coupled to a reluctance generator/motor, without intervening gear couplings and often employing a common integral drive shaft between turbine and reluctance generator/motor. A part of this evolution has involved equipment called the "More Electrical Aircraft Integrated Power Unit", i.e., the MEA IPU, in the United States Air Force. Apparatus of this type is usually operated at a high revolution rate since considerations involving size and weight are as important as operating life, maintenance requirements and operating noise levels in such equipment. A temperature maintenance arrangement for one portion of this apparatus, the reluctance generator/motor portion, is the subject of the present invention.
As noted, the switched reluctance generator/motor is also found desirable for use as a two-way energy transducer in an aircraft propulsion jet engine; i.e., for use during starting of the engine and then for use in converting a portion of the engine's mechanical energy output into the aircraft's electrical energy source. In the United States Air Force, this embodiment of the switched reluctance generator/motor has been given the name of an "Integral Starter/Generator". In this propulsion jet engine application, a somewhat larger and slower-rotating switched reluctance generator/motor than is optimum for the Aircraft Integrated Power Unit application is needed; nevertheless the energy densities involved in this use are also sufficient to make cooling, as in the present invention, an important consideration in the energy transducer's configuration.
In each of these aircraft uses it is desirable to operate a switched reluctance generator/motor at such energy densities and rates of revolution, rates in the forty thousand to seventy thousand revolutions per minute range for example, that energy losses within the machine's enclosure can result in harmful temperature increases in machine components. Notwithstanding common use of cobalt steels and the thinnest (e.g., three mills) laminations practical for example, it is possible for the operating temperature of the rotor in a reluctance motor/generator of such equipment to approach operating temperatures of one thousand degrees Fahrenheit. At this temperature the metallurgical and magnetic properties of the rotor material are often adversely affected. In the winding-free rotor arrangement of a reluctance motor/generator, energy losses causing these temperatures arise not from copper or I.sup.2 R sources, but from hysteresis and eddy current losses in the magnetic material and also from windage losses attending the rotor operation at high rates of revolution.
A plurality of cooling arrangements have been employed in previous high performance reluctance motor/generator devices. One such cooling arrangement is shown in the companion patent application of my colleague, Mr. Edward Durkin, U.S. patent application Ser. No. 08/653,035, Filed May 24, 1996, now U.S. Pat. No. 5,703,421, titled RELUCTANCE GENERATOR/MOTOR COOLING. The contents of this Durkin patent are hereby incorporated by reference herein. Some additional previous cooling arrangements for such machines involve the use of flowing liquids traversing one or more parts of the machine, such as within the machine winding conductors (as shown in U.S. Pat. No. 5,489,810 for example) or within the machine magnetic elements (as shown in U.S. Pat. Nos. 5,189,325 and 3,681,628 for example), while others employ the biphase fluid heat exchanger device which is otherwise known as a heat pipe.
Although such cooling arrangements can be effective, it is presently a desire of the U.S. military to exclude liquids (and moreover even lubricant liquids) from the environment of reluctance motor/generator electrical machines used for certain purposes in an aircraft and to rely instead on gaseous fluids, especially air, for machine cooling purposes. Moreover this desire also extends to an avoidance of gearing in the machine's mechanical energy input circuit and thereby includes the elimination of lubricants needed in the machine's mechanical input circuit. The removal of cooling oil for example from use in such machines can be appreciated to offer significant benefits with regard to simplified equipment maintenance procedures, increased battle damage immunity and fire safety when the machine is used in a combat aircraft. The present invention considers one aspect of this electrical machine enhancement effort, the improved control of gaseous coolant fluids in an open rotor or salient pole rotor reluctance electrical machine.